When a reactor’s power supply is interrupted, it may affect the system’s ability to cool the plant.

To prevent accidents, the outer wall of reactor containment systems are made out of reinforced concrete and steel. Since they are designed to withstand the impact of a large commercial airliner, flying debris – even if it’s propelled by 200 mph winds – is unlikely to pose much of a threat.

Therefore, utilities prepare for storms by inspecting power stations, securing equipment, testing backup pumps and generators and stocking critical supplies in case workers have to stay on site.

The plant, located 25 miles south of Miami, was subjected to sustained winds of 145 mph with gusts up to 175 mph.

While the reactors themselves were not damaged, the plant site sustained US$90 million worth of damage. It lacked external power for five days, relying on backup generators to run critical equipment and keep the reactor cores cool.

A Nuclear Regulatory Commission’s report on the incident noted that the plant had begun to shut down 12 hours before the storm arrived, earlier than required at the time.

Had the plant operators strictly adhered to the requirements, the plant may not have been ready to take the necessary precautions once the storm hit. As a result, plant operators today begin their shutdown procedures and status reports to the NRC 12 hours ahead of the storm’s impact.

3. Why do utilities sometimes wait before turning reactors back on?

At any given moment, the amount of power electricity grids generate must equal the amount customers consume plus what gets lost on its way to them. When there is no way to consume the power, or to transmit it, utilities halt generation.

And even when utilities take steps to protect the grid, such as by laying power lines underground to reduce the risks posed by downed trees and flying debris, it makes them more susceptible to storm surges and flooding.

Therefore, when large numbers of power lines and substations are disrupted, reactors once turned off may not be able come back on line until after all that infrastructure has been repaired.

Before Hurricane Irma made landfall in South Florida in September of 2017, Florida Power and Light originally planned to shut down the Turkey Point reactors 24 hours in advance of landfall, but ultimately made the decision to leave one of them online as Irma’s path changed.

The disaster began when a tsunami caused by the Tohoku earthquake disabled emergency generators used to cool nuclear reactors, causing multiple meltdowns, followed by explosions and the release of radioactive material.

The company also predicted before the hurricane that as many as three-quarters of its 4 million customers in the Carolinas could lose power in outages that could last for weeks – depending on the storm’s severity and trajectory.